Marine heatwaves detected by a network of buoys along the entire US West Coast Shelf

Marine heatwaves (MHWs), prolonged periods of anomalously warm waters, pose an increasing threat to marine ecosystems globally. While satellite-based data offer insights into MHW patterns, coastal upwelling regions are prone to satellite biases and require long-term in-situ datasets for accurate characterization. Using two decades of temperature data from 22 surface buoys distributed along the US West Coast continental shelf (10-50 km offshore) spanning nearly 2,000 km of coastline, we investigated, for the first time, coast-wide patterns of MHWs in this valuable region. The spatial extent of MHWs varied from 100% of buoys during the 2014-2016 North Pacific MHW, to less than 50% during regional events, to single buoys, highlighting the broad range of MHW scales and drivers. Upwelling was a strong contributor to MHW patterns, both on seasonal time scales with less MHWs during the spring/summer upwelling season, as well as synoptic time scales with the initiation and termination of MHWs linked with anomalously weak and strong upwelling, respectively. This was especially evident in central and northern California, where upwelling intensity and variability were stronger and regional co-occurrence lower, than in southern California and Oregon/Washington, where upwelling was weaker and regional co-occurrence higher. Furthermore, there were almost no long-term trends in MHW metrics along the coast, suggesting this region could serve as a thermal refugia in a warming ocean. Effective management and forecasting of coastal MHWs in this region will require capturing a broad range of spatial scales and drivers, with site-specific studies and/or high-resolution models important in many cases. • Marine heatwave analysis across entire US West Coast using in-situ buoy network • Broad range of marine heatwave extent, ranging from entire coast to single buoys • Strong seasonality with less marine heatwaves during upwelling season • Initiation (termination) of heatwaves linked to anomalous weak (strong) upwelling • Almost no long-term trends in marine heatwaves indicates thermal refugia